use HG10UN header for uncompressed bundle
- use HG10UN instead of HG11 for uncompressed bundles header
- check HG10BZ for compressed bundle
- better error handling for invalid header
some notes:
- people who created uncompressed bundle will no longer be
able to use them (it could be fixed with hand-editing)
- older hg cannot detect an uncompressed bundle
(bzip2 decompression will fail).